There is a simpler explaination to this: If you spend a few hours doing some exercise (changing cloth, showering, going to the sport area) you are not sitting alone, as usual in front of a display or in your couche. It distracts your mind from your negative thoughts. Also beeing able to walk normally and without pain, going up stairs etc. is a good and normal feeling. When you get older and those basic things stop working, that really sucks. Or being overweight when 'normal' things become tedious.